PROCESS AND DEVICE FOR TREATING WASHING LIQUID IN PRINTING PRESSES, PARTICULARLY FOR OFFSET PRINTING PRESSES

Abstract: The present invention refers to a process and a device for treating the washing liquid in printing presses, particularly for offset printing presses. The treatment process consists of subjecting the washing liquid, which is based on water and has preset levels of acidity and electrical conductivity, to electrolysis before it is sent to the washing cylinder of the printing press. This process makes it possible to increase considerably the speed at which the ink dries in the printing phase and to enhance the efficiency of the printing press significantly.

Description: Process and Device for Treating Washinq Liquid in Printing Presses, Particularlv for Offset Printing Presses The object of the present invention is a process and a device for treating the washing liquid in printing presses, particularly for offset printing presses. As we know, the efficiency of printing presses, especially offset printing presses, is related mainly to the speed at which the ink used for the printing dries. The task of increasing the ink drying speed is thus a particularly acute problem in this area. To carry out printing by the so-called "offset" method, the cylinder that holds the plate is inked by the inking cylinder and does the printing on a rubber cylinder which, in turn, transfers the printing to the sheet held by the printing cylinder. The inking cylinder, in addition to being coated with ink, is "bathed" with a liquid that is usually composed simply of water, or a mixture of water and alcohol. In the kind of printing that uses water as a washing liquid, therebis the problem of low drying speed; which makes it impossible to reach high hourly production rates. In addition, the use of water causes smearing of the plate, resulting in poor-quality printing. The kind of printing that uses water plus alcohol as a washing liquid can achieve considerably better production rates in terms of both quality and quantity since alcohol, by reducing the surface tension of the water, makes it possible to distribute a finer layer of liquid and ink over the plate and thus ensures fewer smearing problems and a faster drying speed, facilitated by the volatility of the alcohol. Along with these advantages, the use of water and alcohol as a washing liquid poses some problems. As a matter of fact, the alcohol dilutes the ink, reducing the contrast effects in printing. Moreover, an ink blurring effect that considerably degrades the quality of the printing is very commonly encountered. Another problem is posed by the need for the machines with water-alcohol washing to have devices for cooling the alcohol in order to keep it from evaporating excessively. An additional problem is the reduced tolerance of the personnel operating these machines for the alcohol vapors. The primary task of the present invention is to solve the problems outlined above by devising a process and a device for treating printing-press washing liquid that make it possible to eliminate the use of alcohol in the washing liquid without impairing, and in fact directly increasing, the drying speed and thus the efficiency of these machines. Within the framework of this task, one goal of the invention is to develop a process that makes it possible to achieve a printing quality that is better than that which can be obtained using water and alcohol as a washing liquid. Another goal of the invention is to propose a process that avoids the smearing problems which arise from using water alone as a washing liquid. A further goal of the invention is to propose a process that is simple to implement and that does not create problems of work environment contamination. This task, as well as those and others that are presented in greater detail below, are accomplished by means of a process for treating the washing liquid in printing presses, particularly offset printing presses, wherein said process consists of subjecting the washing liquid, which is based on water and has preset levels of acidity and electrical conductivity, to electrolysis. The process of the invention can be easily implemented using a device, wherein said device consists of a treatment tank, means of supplying this treatment tank with a washing liquid that is based on water and has preset levels of acidity and electrical conductivity, and a direct-current [d.c.] electric power supply that is connected to an anode and a cathode that are immersed in the washing liquid contained in this treatment tank for electrolysis of the washing liquid placed in this treatment tank. Other characteristics and advantages of the invention are presented in greater detail in the description in the form of a preferred, but not exclusive, embodiment of the process, as well as of the device of the invention, which are illustrated as examples and without limiting their scope, in the attached drawings, where: Fig. 1 is a schematic of a device for carrying out the electrolysis of the process of the invention; Fig. 2 shows a scheme for applying the device to implement the process of the invention to a printing press; Fig. 3 shows another scheme for applying the device in which the electric power supply is composed of a d.c. generator with its poles attached to the treatment tank and to a washing roller. The process of the invention basically consists of using, as the washing liquid, water that is desalinated and demineralized in advance by means of, e.g., a reverse-osmosis filtration process, and to this water are added acidity adjusting agents and electrical-conductivity adjusting agents in such a way as to bring the acidity and electrical conductivity of the washing liquid to preset values. These preset values basically consist of a pH of between 4.5 and 5.5 and an electrical conductivity of between 800 sss and 2200 jiS. The acidity and electrical conductivity can be adjusted independently of one another by using, for example, an acidity adjusting agent such as H2SO4, or HF, or citric acid, and an electrical conductivity adjusting agent such as NaCl, or by using concentrated acid salts simultaneously. The washing liquid with this level of acidity and electrical conductivity value can then be subjected to electrolysis using a graphite anode, or a carbon anode, or a platinum-plated titanium mesh anode, and at least one steel cathode, preferably a stainless-steel one. The power-supply voltage of the anode and the cathode are varied in inverse proportion to the conductivity of the washing liquid and can vary essentially between 58 V and 38 V. In practice, with a washing-liquid conductivity equal to 800 pS, a voltage of 58 V is used, and this voltage is progressively reduced as the conductivity of the washing liquid increases until it is brought to a value of 38 V, at which point the washing liquid reaches an electrical conductivity equal to 2200 pS. The washing liquid is preferably recirculated in a closed circuit from an electrolytic treatment tank to the washing tank mounted on the press, and vice versa. To implement the above-described process, a device is employed which can be readily composed, as shown in particular in Fig. 1, of a treatment tank 1, to which is sent the washing liquid based on water with preset levels of acidity and electrical conductivity, as described above. In treatment tank 1 are immersed an anode 2, preferably made of graphite, and at least one cathode 3, which are connected to a d.c. power supply; this power supply can simply consist of a rectifier, of a known type, that is connected to the power grid. Treatment tank 1 can be made of an electrically insulating material or steel and can itself act as the cathode in the electrolysis process that is carried out inside the treatment tank. In addition, this treatment tank can also be the printing press's own washing tank. As Fig. 2 shows, the device for carrying out the process of the invention can consist of a treatment tank la that is distinct from washing tank 5 which is mounted on the printing press and which is connected to the latter via a closed recirculation circuit with a pump 6 for pumping the washing liquid through the circuit. In this case as well, the device consists of a d.c. power supply 4a that is connected to an anode 2a, preferably made of graphite, and at least one cathode 3a, preferably made of stainless steel, which are immersed in the washing liquid of treatment tank la. To keep any impurities from circulating, a weir 8 is placed inside the treatment tank. As we know, in offset printing presses the essential parts include at least one washing roller, described above, which is colloquially referred to as a "bagnino" bath attendant] and which serves to wet the printing plate before the ink is applied to it. Since this wetting can be done with water or, better yet, with the aqueous solution for the treatment for which the process of the invention has been developed, the inventor has also devised a considerably simplified device that can be readily applied to existing machines. In such a simplified device (Fig. 3), the power supply consists of a d.c. generator 4b which is equipped with a potentiometer for regulation and whose shaft rotates as a single piece with one of the different rotating parts of the machine in which the device is used. In this embodiment above-described treatment tank lb serves, more simply, as the washing tank for the printing press, into which above-mentioned wetter or "attendant" 9 extends, as usual; the latter should be made of a conductive material, e.g., preferably chrome-faced steel. The positive pole of the generator is connected to the above-mentioned washing tank which, since it is also made of a conductive material, should be insulated with respect to the outside and thus acts as the anode, while the negative pole is connected to "attendant" roller 9, which serves as the cathode. It is thus possible to implement the process of the invention, with the same advantages as described above, using a simpler and cheaper device. The electrolysis that is applied to the washing liquid with controlled levels of acidity and electrical conductivity causes an increase in the concentration of oxygen in the washing liquid, which is identified as the factor that causes the ink, in the printing phase, to dry in considerably shorter times than are required for drying in traditional kinds of printing that use just water or water mixed with alcohol as washing liquids. This phenomenon is based on the fact that the inks which are currently being used for printing, particularly for offset printing, have an elevated capacity for absorption and oxidation. Because of the electrolysis to which it is subjected, the heavily oxygenenriched washing liquid makes it possible for the ink to absorb large quantities of oxygen and thus causes it to oxidize faster and thus to dry more quickly. In the case where sodium chloride is used as the agent for adjusting the electrical conductivity of the washing liquid, another factor that accounts for the faster drying that is achieved with the washing liquid that is treated with the process of the invention is the fact that, during the electrolysis process, sodium peroxides are formed which subsequently enhance the oxidizing effect of the washing liquid on the ink. In addition, since the plate that is used for offset printing is generally anodized, the washing liquid, which is rich in ions due to the electrolysis process, adheres better to the plate, which makes it possible to have a constant and much thinner layer of liquid and ink on the plate, thus avoiding smearing problems. It has been confirmed in practice that the process of the invention completely accomplishes the task at hand since it makes it possible to ensure considerably faster drying of the ink used for printing than can be achieved with the traditional use of water or water mixed with alcohol. Another advantage lies in the fact that, with the process of the invention, the use of alcohol in the washing liquid is eliminated, resulting in considerably better printing quality. The elimination of alcohol from the washing liquid also makes it possible to simplify the printing press since the devices for cooling the alcohol are no longer needed and hazardous substances are no longer released into the work environment. As designed, the process, as well as the device for its implementation, can have many modifications and variations, all within the scope of the inventive concept; in addition, all of the parts can be replaced with other parts that are technically equivalent.

Claims: Claims 1. A process for treating the washing liquid in printing presses, particularly for offset printing presses, characterized in that said process consists of subjecting the washing liquid, which is based on water and has preset levels of acidity and electrical conductivity, to electrolysis. 2. Process according to claim 1, wherein the abovementioned washing liquid consists of desalinated and demineralized water with the addition of acidity adjusting agents and electrical-conductivity adjusting agents. 3. Process according to one of claims 1 and 2, wherein the washing liquid has a pH that lies essentially between 4.5 and 5.5. 4. Process according to one of the preceding claims, wherein the washing liquid has an electrical conductivity that lies essentially between 800 4S and 2200 pS. 5. Process according to one of the preceding claims, wherein the acidity adjusting agents are sulfuric acid. 6. Process according to one of claims 1-4, wherein the acidity adjusting agents are hydrofluoric acid. 7. Process according to one of claims 1-4, wherein the electrical conductivity adjusting agents are citric acid. 8. Process according to one of the preceding claims, wherein the electrical-conductivity adjusting agents are sodium chloride, which promotes the formation of sodium peroxides in the electrolysis bath. 9. Process according to one of the preceding claims, wherein the electrical-conductivity and acidity adjusting agents are concentrated acid salts. 10. Process according to one of the preceding claims, wherein the electrolysis of the above-mentioned washing liquid is done using a graphite anode and at least one steel cathode. 11. Process according to one of the preceding claims, wherein the power-supply voltage of the anode and the cathode are varied in inverse proportion to the conductivity of the washing liquid and can vary essentially between 58 V and 38 V. 12. Process according to one of the preceding claims, wherein the washing liquid is recirculated in a closed circuit from an electrolytic treatment tank to the washing tank mounted on the press, and vice versa. 13. Device for treating the washing liquid in printing presses, particularly for offset printing presses, wherein said device consists of a treatment tank (1, la, lb), means of supplying said treatment tank with a washing liquid, which is based on water and has preset levels of acidity and electrical conductivity, and a d.c. power supply (4a) that is connected to an anode (2a) and a cathode (3a) in contact with the liquid contained in this treatment tank for electrolysis of the washing liquid in this treatment tank. 14. Device according to claim 11, wherein abovementioned anode (2a) is made of graphite. 15. Device according to claim 13, wherein abovementioned anode (2a) is made of carbon. 16. Device according to claim 13, wherein abovementioned anode (2a) is made of platinum-plated titanium mesh. 17. Device according to one of claims 13-16, wherein above-mentioned cathode (3a) is made of steel. 18. Device according to one of the preceding claims, wherein above-mentioned treatment tank (la) is made of an electrically insulating material. 19. Device according to one of the preceding claims, wherein above-mentioned treatment tank (la) is made of steel and constitutes the above-mentioned cathode. 20. Device according to one of the preceding claims, wherein above-mentioned d.c. power supply (4, 4a) consists of a rectifier powered from the power grid. 21. Device according to claim 13, wherein abovementioned treatment tank (lb) consists of the washing tank of the printing press and wherein the power supply is a d.c. generator (4b), whose positive pole is connected to washing tank (lob) made of a conductive material and whose negative pole is connected to an "attendant" roller (9), which is made of a conductive material and extends into the liquid contained in washing tank (lb) itself. 22. Device according to claim 21, wherein "atten dant" roller (9) is made of chrome-plated steel. 23. Printing press, particularly an offset printing press, wherein said printing press includes a device for treating the washing liquid as described in one of claims 1122.

Inventors: GUERINI ARTURO, GUERINI, ARTURO
Applicants: GUERINI ARTURO, GUERINI, ARTURO
Application year: 1994
Main classification: B41F7
IPC codes: B41F7, B41N3, C02F1